The present invention relates to power conversion and more specifically to an improved AC--AC power converter for use in conjunction with AC loads having high power requirements.
AC--AC converters are static power converters which change a fixed voltage and fixed frequency AC power source into an output AC power source having variable voltage and variable frequency (VVVF) in a controlled manner. Unlike a voltage source or current source inverter which will first convert the AC input power to a DC power and then reconvert the DC power back to VVVF AC power output, AC--AC converters perform the AC--AC power change in a one-stage conversion without using any energy storage components in a DC-link. Such AC--AC conversion is a highly efficient power conversion.
In AC--AC PWM converters, self-turn-off static power devices are used to form bi-lateral switches for each input phase. A circuit of this type which consists of 9 bi-lateral switches S.sub.1 -S.sub.9 is provided in FIG. 1. This circuit is also known as a matrix converter since it provides one switch for each of the possible connections between the input and output AC lines.
In existing AC--AC matrix converters, a bi-lateral switch consists of a current steering diode bridge and a power transistor device as shown in FIG. 1. Although each of the bi-lateral switches has only one switching device and four fast recovery power diodes, special care in the load current commutation is needed. A dead-time insertion is typically employed between turning the switch off in the outgoing phase and turning the switch on in the incoming phase, to prevent a short circuit between the outgoing and incoming phases. The dead-time is typically in proportion to the device current and voltage rating. During the dead-time period, the load is electrically open circuit from the power source due to both the outgoing and incoming devices are off and the normal current flow is interrupted. This causes a high voltage spike across the switching devices due to the load and line side inductances. Large sized snubber capacitors are needed to suppress the transient voltage spikes to protect the switching devices. This increases the converter switching losses and unit cost.
Another drawback of the existing configuration of the three-phase matrix converters is that their output power rating is dictated and limited by the commercial availability of the power switching devices.
Alternatively, FIG. 2 shows a power converter topology which is able to provide equivalent functions by the combination of a PWM rectifier bridge and a PWM inverter bridge circuit. The DC link capacitor values can be reduced by coordination of the PWM rectifier and PWM inverter switching. However, there exists an explicit DC link stage in this power circuit. The output power rating is also limited by the commercial availability of the power devices.
Practically, due to the limits of the device ratings of components used for the forced commutation PWM converters in FIGS. 1 and 2, it is not possible to produce feasible converters which meet very high power (high voltage and high current) requirements. For instance, known switching techniques in forced commutation employ IGBT devices. While use of such devices have many benefits, IGBT based conventional power converter design is commercially practicable only within a limited power rating. This is due to the IGBT and associated passive components which can withstand higher voltage and current, for example for 920 V to 6600 V application, are either not readily available or not cost effective.
Additionally, in many industrial settings products will have power ratings which vary from a low to a very high power. For example, in an industrial setting a load may be a 400 kilowatts machine, however, in another industrial setting a piece of equipment may require over 10,000 kilowatts to operate. Therefore, for a manufacturer to supply such a wide range of requirements it is necessary to have a wide variety of AC--AC converters with different power ratings. Such a situation is very expensive not only due to the construction costs of so many varied units but also in maintaining and stocking so many different components. Particularly, each different unit needs to be individually designed, and thoroughly tested and debugged. Further expenses are incurred due to developmental cost and design time which each unit needs. All the above drawbacks raise the cost of supplying such converters.
It is therefore an object of the present invention to provide a multi-modular structure which increases the ease of development, manufacture, testing, and field service by utilizing commonly available and field proven power components having low voltage ratings to replace a wide variety of and expensive high voltage active power devices and passive components.
It is a further object of the present invention to provide direct AC--AC power conversion with low switching losses, thus increasing the conversion efficiency and reducing the physical size of the converter, thereby eliminating the front end rectifier, DC-link and precharge circuitry in a traditional AC-DC-AC power conversion.
It is yet another object of the present invention to provide inherent power regeneration for true four quadrant operation of an AC drive, provide, especially for large horse power requirements, low harmonic distortion on both line side and motor side.